There are many different types of storage devices for computer systems on the market; each type having at least some differences in the way it interfaces to a computer system. Often, a single company markets more than one type of mutually incompatible storage device. For example, consider Redundant Array of Independent Disk (RAID) storage devices, as known to be available from many vendors including EMC and Compaq. RAID storage devices are known that may operate in any of several modes, including mapping of single or concatenated disks and disk sets to logical volumes without redundancy. Other modes may include mapping datasets to disk sets while maintaining data parity across multiple disks such that data is redundant and can be rebuilt if any one drive fails (RAID-4 and RAID-5 modes). RAID storage devices may have multiple controllers where each controller can access a set of common disk drives. RAID and other storage devices may also be operated in local or remote mirror-set modes, where duplicate copies of data are maintained in separate sets of disk drives.
Dual-controller RAID systems are known, including some Compaq Raid Array 8000 systems, wherein there may be two controllers sharing access to an array of disk drives. Systems of this type are available that operate in a active/standby redundant mode, where one controller provides access to all logical units of storage provided by the system. In active/standby mode, failure of the active controller causes failover to the standby controller, which then provides access to the logical units. Systems of this type are also known that operate in an active/active mode, where both controllers are normally active. In active/active mode, each controller normally provides access to a subset of the logical units of storage provided by the system, and failure of either controller causes failover of those units it was serving to the remaining controller.
In dual-controller RAID systems of either type, each controller typically has its own set of SAN addresses. It is therefore necessary for each storage node accessing the dual-controller system to automatically change the SAN addresses used for accessing at least some LUNs of the RAID system from controller to controller as failovers occur. Further changes to SAN addresses for particular LUNs may also be required as controllers are repaired, or system throughput optimized as loads are balanced between dual active controllers.
Failover (a taking-over by standby hardware of storage-serving activity, typically necessitated by failure of active hardware) often can be induced either by failures detected by a controller of the dual controllers, or by failures detected by a compute node. When a compute node detects failure of, or inability to communicate with, a particular controller, it may issue commands instructing the other controller of the same dual-controller system to take over the storage-serving activity formerly handled by the failed controller. It is known that these failover commands may differ between controllers of different types, even if those controllers are made by the same manufacturers.
Existing RAID and other redundant storage devices typically require control functions and control sequences for controlling their redundant features in addition to typical read and write control functions. Further, multiple-controller RAID devices are known that can transfer responsibility for particular datasets from one controller to another upon failure of a controller or otherwise. These redundancy control functions may include, in addition to other functions:    a. monitoring of one or many connections between a computer and storage controller, such that data is sent over properly operating interconnect,    b. monitoring of controller access to particular datasets,    c. monitoring of controller activity and health, such that data is communicated between computer and functioning controllers that have access to the data desired;    d. determining when to split, and when to re-synchronize, mirrored data,    e. performance of dataset reconstruction and re-synchronization as needed,    f. coordinating redundancy control functions with any other computers having access to the data to avoid conflicts that can cause data loss, such as performing incompatible updates to different copies of split mirrored datasets,    g. controlling failover of individual LUNS among controllers, and/or entire redundant controllers, to provide continued access to datasets, and    h. balancing loads among active/active redundant controllers.
RAID and other storage devices typically connect to computers through a form of interconnect and a host adapter of each computer. The interconnect often complies, and operates according to, a standard for cabling and protocol such as a version of the SCSI (Small Computer Systems Interface) standard or the Fibre Channel standards.
Multiple computer systems may connect to multiple storage systems through a Storage-Area Network (SAN). A storage area network (SAN) is a dedicated, centrally managed, information infrastructure, which enables interconnection of compute nodes and storage nodes. A storage area network facilitates universal access and sharing of storage resources. SANs are often implemented with Fibre Channel technology as known in the art. Typically, a SAN utilizes block-oriented protocols, such as a SCSI-like protocols encapsulated within Fibre Channel frames, for providing storage to compute nodes. This differs from general purpose networks (GPNs), including local area networks (LANs), wide area networks (WANs) and the Internet, which typically implement file-oriented protocols. Some storage area networks may encapsulate block oriented protocols in other protocols, a protocol of this type is iSCSI.
Connection of RAID and other storage devices to a computer system through a SAN further complicates control of redundancy features because it is often possible to find more than one path through a SAN from the computer to one or more of the storage devices. When more than one such path exists, it is necessary to control the path over which data is communicated, and it may become possible or desirable to distribute communications among the multiple paths.